<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        let radius = prompt("请输入圆的半径:");
        let area = Math.PI * radius * radius;
        document.write("圆的面积是: " + area);
    </script>
</body>
</html>